1. Field of the Invention
The present invention relates to a device which allows monitoring of the level of the condensate collected in the measuring cylinder of an automatic standardized device for distillation of liquid samples, in particular samples of petroleum products under atmospheric pressure.
Such a distillation device allows the measurement of the distillation parameters of these samples to be carried out, while complying with a predefined test standard selected from a plurality of possible test standards.
2. Description of the Related Art
It is known that the distillation parameters of petroleum products are representative of the performance levels of these products and the risks which may be involved for those who use them.
The determination of these parameters is particularly significant in the case of fuels which are intended for the automotive industry or aviation where problems relating to safety are of prime importance.
These parameters are in particular tables or lines representing the percentage of a sample evaporated in accordance with the temperature during a distillation or the volume of the residue and the losses.
Specialists can deduce from these parameters the behaviour of a specific petroleum product in a specific situation and therefore determine whether or not this product can be safely used, in order to obtain the desired performance levels.
In this context, specialists have stipulated various test standards which define very precisely the conditions under which such distillation characteristics must be obtained.
Consequently, in order to provide usable results, the distillations must be implemented with these standards being strictly complied with.
Various automatic distillation devices are currently commercially available and allow the distillation parameters of a liquid sample to be measured, while complying with a predefined test standard.
These standardized distillation devices generally comprise:                a fixed frame,        a heating chamber which comprises a heat-generating element, in particular a heating resistor,        a series of distillation flasks which correspond, respectively, to at least one test standard and which are capable of being fixed to the frame of the device in a fixed position, the column of these flasks being able to be closed by means of a fluid-tight closure stopper which is provided with a thermometer allowing the temperature of the evaporated vapours to be measured and which comprises a lateral branch which is intended to be connected to a condensing tube,        a series of insulating plates which are intended to be mounted above the heat-generating element in order to close the heating chamber at the upper portion thereof and which are each provided with a central opening having a geometry which is adapted to that of the base of an associated distillation flask,        a measuring cylinder which allows the condensate to be collected and which is provided with means for measuring the quantity of condensate collected in this manner, and        control and regulation means which allow an operating variable of the heat-generating element to be controlled and varied over time, in particular the temperature or the power of this element in order to obtain distillation parameters in accordance with a predefined test standard.        
So that the distillation parameters of a sample obtained are completely representative of this sample, it is essential for the measuring means with which the measuring cylinder is provided to be capable of allowing very precise determination of the level of condensate collected in this cylinder and very precise monitoring of this level over time.
To this end, it has already been proposed to provide standardized distillation devices of the above-mentioned type with means for measuring the quantity of condensate collected in the measuring cylinder comprising an optical system which is constituted by a transmitter/receiver pair which is capable of transmitting an infrared beam which is directed horizontally through the measuring cylinder and co-operating with a linear driving action via a step motor which can be moved in vertical translation in order to allow this beam to be aligned with the meniscus of the condensate collected in this cylinder in order to allow the height of this meniscus to be detected.
In such distillation devices, the infrared beam may or may not be centered on the center axis of the measuring cylinder.
The linear driving by means of a step motor can be very precise in order to allow the infrared beam to be moved by a pitch in the order of 0.05 mm in the vertical plane.
Such measuring means which are based on transmitting and receiving an infrared beam further have the advantage of not being affected by ambient light.
However, it should be noted that an eccentric light beam, which passes between the center and the wall of the measuring cylinder, allows a clear distinction to be made between the liquid sample to be analyzed and the air located above this sample in so far as, when it is located below the level thereof, it is interrupted and can no longer reach the detector.
However, such an eccentric light beam does not allow it to be determined whether the location is below the lower portion of the meniscus, that is to say, the “true” meniscus, or between the lower portion and the upper portion thereof.
An eccentric light beam consequently only allows the upper portion of the meniscus to be detected so that, in order to obtain the “true” meniscus, it is necessary to carry out a correction which is dependent on the sample and is therefore imprecise.
A central light beam itself has the disadvantage of not allowing a sufficiently reliable level measurement to be obtained, in particular at the beginning of distillation where, taking into account phenomena of capillarity, the meniscus of the condensate is not completely constituted with the result that it is impossible to detect the lower portion thereof.
Furthermore, a center beam is not interrupted by the liquid sample but instead the intensity thereof is only reduced.
Such a beam can therefore allow the upper portion and the lower portion of the meniscus to be detected, that is to say, the “true” meniscus, but in the case of some samples, the reduction of the intensity of the beam under the effect of air or liquid may be of the same magnitude, so that it is impossible to determine whether the location is above or below the liquid level; such samples cannot consequently be detected.